The present invention relates to heat exchangers and, more particularly, to combine evaporative and non-evaporative heat exchangers. The evaporative section of the heat exchanger may have both a direct and indirect portion.
Waste heat from industrial processes such as condensers or heat exchangers may itself be released to the atmosphere by non-evaporative or so called dry heat exchangers. In such non-evaporative heat exchanger and air stream is in indirect contact with a process fluid stream. In a close system, the process fluid stream is enclosed so that there is no direct contact between the air stream and the process fluid stream. The enclosing structure is usually a coil of tubes. Heat is exchanged as an air stream is passed over the coil structure enclosing the process fluid stream.
Waste heat may also be rejected to the atmosphere by evaporative heat exchangers which offer significant process efficiency improvements over non-evaporative heat exchangers. One type of evaporative heat exchanger is a direct evaporative direct heat exchanger. In a direct evaporative heat exchanger, an air stream is in contact with an process fluid stream. The process fluid stream is usually water and the two streams come into direct contact with each other.
Another type of evaporative heat exchanger is an indirect close circuit evaporative heat exchanger wherein an air stream passes over an enclosed process fluid stream while an evaporative liquid also passes over the enclosed process fluid stream. The enclosed fluid exchanges heat with the evaporative liquid through indirect heat transfer, since it does not directly contact the evaporative liquid and then the air stream.
Such combined evaporative and non-evaporative heat exchangers consume energy in the form of electricity for fan and pump operation and water during the process of rejecting heat. It is desirable to operate such combined evaporative and non-evaporative heat exchangers in an efficient matter to minimize the combined consumption of energy and water. Heat rejection equipment must be selected for the maximum heat loaded summer peak air temperatures. In combined evaporative and non-evaporative heat exchangers, it is desirable to operate such heat exchangers as efficiently as possible. To date, control mechanisms for such operation have not addressed both energy savings and water savings.